Shaft member with vibration damping function

ABSTRACT

Provided is a shaft member capable of minimizing the occurrence of noise by positively attenuating vibration acting on a metal shaft member having spline grooves and spiral grooves axially formed therein. To attain such an object, a shaft member includes a metal shaft main body having a hollow portion and a ceramic material serving as a vibration absorbing member which fills the hollow portion of the shaft body with no gaps. The ceramic material is filled into the hollow portion of the shaft main body in a powder, granular, or fluid state, and formed within the hollow portion into a shape matching the hollow portion.

TECHNICAL FIELD

The present invention relates to a shaft member made of metal, in whicha spline groove, a helical groove, and the like are formed along anaxial direction thereof. In particular, the present invention relates toimprovements in order to attenuate vibrations or noise at an earlystage.

BACKGROUND ART

For example, there are known conventional ball splines that include aspline shaft in which ball rolling grooves are formed along an axialdirection, and a spline nut that fits together with the spline shaftthrough endlessly circulating balls, in which the spline nut is free tomove in the periphery of the spline shaft along the axial direction andin which torque transmission between the spline shaft and the spline nutis possible.

Further, there are known conventional ball screw devices that include ascrew shaft with helical ball rolling grooves formed at a predeterminedpitch therein, and a screw nut that fits together with the screw shaftthrough endlessly circulating balls, in which the screw nut moves in alongitudinal direction according to rotation of the screw shaft.

However, the following problem may occur when using the ball screwdevices, the ball splines, or the like. The balls circulate endlesslyaccording to relative motion between the nut and the spline shaft or thescrew shaft. This invites a result in which the balls vigorously impactthe shaft member when the nut moves at high speed because the ballsseparate from and contact the shaft members in turn. The shaft membersare vibrated due to the impacts, and a grating oscillatory sounddevelops.

Further, these types of ball screw devices and ball splines are oftenused in the Z-axis of industrial robots and the like. However, in orderto position the Z-axis with high precision, it is necessary to attenuatevibrations that act on the spline shaft or the screw shaft at an earlystate. If the settling time for the vibrations to subside is long, thereare problems in that the tact time for completing one work unit becomeslonger, and the manufacturing efficiency decreases.

Conventionally, a hollow shaft disclosed in JP 62-103133 A is known as ashaft member made in view of problems like those described above. Acircumferential wall of the hollow shaft is formed by laminating anintermediate layer of a ceramic material between an inner layer and anouter layer that are made of a metal; and does not easily transmitvibrations.

There are problems, however, in that the hollow shaft with the laminatestructure is laborious to fabricate, and manufacturing costs pile up.Further, vibrations acting on the shaft member differ according to theshape and the intended application of the shaft member itself. In orderto effectively attenuate the vibrations, it is necessary to optimize thelaminated intermediate layer according to vibration frequency and thelike. Regarding this point, the intermediate layer is formed thinly withthe laminated hollow shaft. Accordingly, there is a problem in thatthere is little breadth in the design and selection of the intermediatelayer, and it is difficult to effectively attenuate vibrations for alltypes of applications.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of problems like thosedescribed above. An object of the present invention is to provide ashaft member capable of positively attenuating vibrations acting on theshaft member, and capable of suppressing generation of noise as much aspossible.

In order to achieve the objects described above, the shaft member of thepresent invention is characterized by including a shaft main body madeof metal and provided with a hollow portion, and a ceramic materialserving as a vibration absorbing member and filling the hollow portionwith no gaps.

While the ceramic material may be formed into a predetermined shape andthen pushed into the hollow portion of the shaft member under pressureconsidering the ease of assembling of the shaft member, it is preferablethat the ceramic material be filled within the hollow portion of theshaft member in a powder, granular, or fluid state, and be formed withinthe hollow portion into a shape that conforms to the hollow portion.

In addition, it is preferable that the ceramic material be a hydrauliccomposition having a hydraulic powder and a non-hydraulic powder as itsmain components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view that shows an embodiment of a tie rod (shaftmember) applying the present invention.

FIG. 2 is a schematic view that shows a configuration of a steeringapparatus using the tie rod shown in FIG. 1.

FIG. 3 is a cross sectional view taken along a line segment III-III ofFIG. 1.

FIG. 4 is a perspective view that shows a ball screw spline in which thepresent invention is applied to an output shaft.

FIG. 5 is a cross sectional view that shows an example of constructing arobot arm using the ball screw spline shown in FIG. 4.

FIG. 6 is a surface view that shows an embodiment of a screw shaft of aball screw device applying the present invention.

DESCRIPTION OF SYMBOLS

8 . . . tie rod (shaft member), 8 a . . . hollow portion, 8 b . . .shaft main body, 8 c . . . vibration absorbing member

BEST MODES FOR CARRYING OUT THE INVENTION

A shaft member provided with a vibration damping function according tothe present invention is explained in detail below based on the appendeddrawings.

FIG. 1 is a drawing that shows an example of a tie rod 8 used in a powersteering apparatus of a vehicle. As shown in FIG. 2, the power steeringapparatus to which the tie rod 8 is applied includes a steering system1, a rack and pinion mechanism 3 that is connected to an output shaft 2of the steering system 1, steering torque detecting means 4 fordetecting a steering torque of the steering system 1, controlling means5 for generating a control signal based on a signal detected by thesteering torque detecting means 4, an electric motor 6 that generates anauxiliary torque in accordance with the steering torque based on thecontrol signal of the controlling means 5, and a steering ball screw 7that threadedly engages with the tie rod and is imparted with rotationby the electric motor 6. When a steering wheel 1 a is operated, the tierod 8 is moved linearly in an axial direction by the rack and pinionmechanism 3 and the steering ball screw 7, and vehicle wheels 11 a and11 b that are connected to both ends of the tie rod 8 are steered.

The steering system 1 comprises the steering wheel 1 a, an input shaft12 that is connected to the steering wheel 1 a, and the output shaft 2that is connected to the input shaft. The steering torque detectingmeans 4 is adapted to detect steering torque, that is, a relative angleof twist between the input shaft 12 and the output shaft 2.

Further, the rack and pinion mechanism 3 includes a pinion gear 11 thatis connected to a distal end of the output shaft 2 through a universaljoint, and rack teeth 9 that are formed in the tie rod 8 and mesh withthe pinion gear 11. When the steering wheel la is rotated, rotation ofthe steering wheel la is transmitted to the pinion gear 11 through theinput shaft 12 and the output shaft 2. The pinion gear 11 then pushesthe tie rod 8, which has the rack teeth 9, in an axial direction.

The tie rod 8 is a hollow shaft provided with a hollow portion 8 a. Therack teeth 9 are formed at one end side of the tie rod 8. A helical ballrolling groove 10 is formed at the other end side of the tie rod 8. Asteering ball screw 7 threadedly engages with the ball rolling groove 10through a plurality of balls that roll in the ball rolling groove 10.The vehicle wheels 11 a and 11 b are connected to both ends of the tierod 8 through a pair of ball joints 13, 13, respectively. When the tierod 8 moves linearly in the axial direction due to operation of thesteering wheel 1 a, the linear motion is transmitted to the vehiclewheels 11 a and 11 b, through the ball joints 13, as rocking motion,thus performing steering of the vehicle wheels 11 a and 11 b.

In addition, in order to smoothly move the tie rod 8 in the axialdirection according to operation of the steering wheel 1 a, the electricmotor 6 generates an auxiliary torque in accordance with the steeringtorque. The steering ball screw 7 is rotated in a predetermineddirection by the auxiliary torque. The tie rod 8 is thus pushed in theaxial direction by not only the rack and pinion mechanism when thesteering wheel la is operated, but also by the steering ball screw 7.

On the other hand, the tie rod 8 is one in which the rack teeth 9 andthe ball rolling groove 10 are formed in an outer circumferentialsurface of the metallic shaft main body 8 b that is provided with thehollow portion. The rack teeth 9 and the ball rolling groove 10 areformed by cutting or by component rolling. While the rack teeth 9 andthe ball rolling groove 10 may be formed in a single shaft main body 8b, it is also possible to form a shaft main body that is provided with ahollow portion by welding together a hollow shaft in which rack teethare formed and a hollow shaft in which a ball rolling groove is formed.

FIG. 3 is a diagram that shows a cross section of the tie rod 8 in thedirection perpendicular to the axial direction. A vibration absorbingmember 8 c is packed into the hollow portion 8 a of the tie rod 8,without gaps, in this embodiment. The vibration absorbing member 8 cfirmly adheres to an inner circumference of the shaft main body 8 b. Thevibration absorbing member 8 c may be integrated with the shaft mainbody 8 b by press fitting a member formed in a cylindrical shape intothe hollow portion 8 a of the shaft main body 8 b. However, consideringthe labor involved in forming the vibration absorbing member 8 c itselfin a cylindrical shape with good precision, and considering thatunevennesses may exist within the hollow portion 8 a of the shaft mainbody 8 b, and the like, it is preferable that the vibration absorbingmember 8 c be filled within the hollow portion of the shaft member 8 bin a powder, granular, or fluid state, and formed within the hollowportion 8 a into a shape conforming to the hollow portion 8 a.

In view of the above, in this embodiment, a hydraulic composition havinga hydraulic powder and a non-hydraulic powder as main constituents (Z-mamanufactured by Sumitomo Osaka Cement) is filled within the hollowportion of the shaft main body with applied pressure. The vibrationabsorbing member, made from a ceramic material, is integrated with theshaft main body by performing hydrothermal synthesis on the filledhydraulic composition. The term hydraulic powder as used here means apowder that hardens with water. For example, calcium silicate compoundpowder, calcium aluminate compound powder, calcium fluoroaluminatecompound powder, calcium sulfur aluminate compound powder, calciumaluminoferrite compound powder, calcium phosphate compound powder,semi-hydrated or anhydrous gypsum, self-hardening calcium oxide, andcompounds of a mixture of two or more of the aforementioned powders canbe used. A powder such as Portland cement can be given, for example, asa typical powder.

Further, the term non-hydraulic powder means a powder that does notharden by itself, even when there is contact with water. Non-hydraulicpowders also include powders whose components are eluted out in analkaline state, an acidic state, or under a high pressure vaporatmosphere, and react with other already eluted components, formingproducts. By adding this type of non-hydraulic powder, it becomespossible to increase the filling ratio when forming a formed product, toreduce the void ratio of the formed product obtained, and to increasethe dimensional stability of the formed product. Calcium hydroxidepowder, gypsum dihydrate powder, calcium carbonate powder, slag powder,fly ash powder, silica powder, clay powder, silica fume powder, and thelike can be given as typical examples of non-hydraulic powders.

The weight composition of the non-hydraulic powder is from 10 to 50% byweight of the mixed powder composed of the hydraulic powder and thenon-hydraulic powder, preferably from 25 to 35% by weight. If the weightcomposition is less than 10%, the filling ratio becomes low, and if theweight composition exceeds 50%, the strength and the filling ratiobecome low. Neither of the weight compositions is desirable for itsadverse influence on the properties obtained after formation andhardening. For example, defects may occur during machining, anddimensional stability may be adversely affected. Considering the ease ofmachining, it is therefore preferable to regulate the weight compositionof the non-hydraulic powder so that the filling ratio does not becometoo low.

Specific processes for integrating the shaft main body and the vibrationabsorbing member are given. First, a mixture obtained by adding 30 partsby weight, or less than the theoretical hydration amount, of water to100 parts by weight of a mixed powder of a hydraulic powder and anon-hydraulic powder is mixed into a mixed powder composed of ahydraulic powder such as Portland Cement, a non-hydraulic powder such assilica fume, and other additives. It is preferable to use a mixingmethod or a mixing machine capable of imparting a strong shear force tothe formation mixture during mixing.

Next, the formation mixture thus obtained is filled into the hollowportion of the shaft main body with pressure. Curing is performed whenfilling is complete. Curing is performed at room temperature or underhigh temperature and high pressure. The curing time changes according tothe curing temperature.

The vibration absorbing member made from the ceramic material can thusfill the inside of the hollow portion of the shaft member without gaps,regardless of the shape of the hollow portion.

Table 1 below shows results on the measured natural frequency, and onthe measured frequency response characteristics under forced vibration,for a hollow shaft and for a hollow shaft with the vibration absorbingmember filling the inside of the hollow shaft, and the damping ratio andthe gain difference (dB) at this time. As shown in Table 1, it wasverified that the damping ratio of the hollow shaft having the ceramicvibration absorbing member filled therein without any gaps is largecompared to the damping ratio of the hollow shaft as it is. TABLE 1Absorbing member Hollow shaft provided 1 Natural frequency (Hz) 421 390Damping ratio 0.0025 0.0145 Gain difference (dB) 0 −17.6 2 Naturalfrequency (Hz) 1136 1079 Damping ratio 0.0008 0.0349 Gain difference(dB) 0 −35.0

Further, as described above, the void ratio and the hardness of theceramic material that fills the hollow shaft can be freely regulated bysuitably changing the weight composition of the non-hydraulic powder andby suitably changing the pressure applied during filling. Accordingly,it is also possible to impart damping performance to the shaft memberaccording to the shape of the hollow shaft and according to thefrequency that acts on the hollow shaft.

Consequently, even if vibrations occur in the tie rod due to engaging ofthe pinion gear and the rack teeth due to operation of the steeringwheel, or due to rotation of the steering ball screw, the vibrations areattenuated at an early stage according to the steering apparatus of thisembodiment. It thus becomes possible to suppress the propagation ofnoise that accompanies this vibration within a passenger compartment ofthe vehicle.

FIG. 4 and FIG. 5 are diagrams that show embodiments of a robot arm thatapplies the present invention.

A ball screw spline and a pair of motors constitute the robot arm.Stroking, rotational, and spiraling motions can be performed on theoutput shaft as an arm by combining rotation and stopping of the pair ofmotors.

As shown in FIG. 4, the ball screw spline is formed by overlapping ahelical ball screw groove 51 and a ball spline groove 52 that extends inan axial direction in an outer circumferential surface of one outputshaft 50. A ball screw nut 53 threadedly engages with the ball screwgroove 51, while a ball spline nut 54 fits into the ball spline groove52. The ball screw spline is designed such that the output shaft 50strokes in an axial direction or rotates by selectively stopping androtating the ball screw nut 53 and the ball spline nut 54. Supportbearings 57 and 58 are therefore assembled onto outer circumferentialsurfaces of the nuts 53 and 54 through a large number of balls 56, 56,respectively, so that the ball screw nut 53 and the ball spline nut 54can rotate freely with respect to a housing to which they are attached.

The ball screw spline is attached to the housing for use as shown inFIG. 5. In other words, the support bearings 57 and 58 of the ball screwnut 53 and the ball spline nut 54, respectively, are fixed to a housing59. The nuts 53 and 54 are supported so as to be free to rotate withrespect to the housing 59. Further, pulleys 60 and 61 are fixed to anend portion of the ball screw nut 53 and an end portion of the ballspline nut 54, respectively. Rotational force from a motor (not shown)is transmitted to the nuts 53 and 54 by timing belts that are hungaround the pulleys 60 and 61.

The ball screw nut 53 used here includes a nut main body 63 in which ahelical load ball groove 62 that opposes the ball screw groove 51 of theoutput shaft 50 is formed in an inner circumferential surface of the nutmain body 63, a pair of end caps 65 that are fixed to both end portionsof the nut main body 63 and assist endless circulation of balls 64, anda large number of the balls 64 that are inserted between the ball screwgroove 51 of the output shaft 50 and the load ball groove 62 of the nutmain body 63. The balls 64 roll between the ball screw groove 51 and theload ball groove 62 accompanying relative rotation between the outputshaft 50 and the ball screw nut 53. Further, a ball return hole 66 isformed in the nut main body 63 along an axial direction thereof, while adirection changing groove (not shown) for sending the balls 64 into theball return hole 66 after the balls 64 finish rolling in the load ballgroove 62 of the nut main body 63 is formed in the end cap 65. The balls64 circulate endlessly accompanying rotation of the ball screw nut 53.

The ball spline nut 54 includes a nut main body 68 in which a load ballgroove 67 that opposes the ball spline groove 52 of the output shaft 50is formed in an inner circumferential surface of the nut main body 68, alarge number of balls 69 that are inserted between the ball splinegroove 52 of the output shaft 50 and the load ball groove 67 of the nutmain body 68, and a ball holder 70 that is fixed to a hollow portion ofthe nut main body 68 and guides and holds the balls 69. The ball splinenut 54 guides the output shaft 50 in the axial direction whilepreventing relative rotation between the output shaft 50 and the ballscrew nut 54.

In addition, the output shaft 50 includes a shaft main body that isprovided with a hollow portion, and a vibration absorbing member thatfills the inside of the hollow portion of the shaft main body withoutgaps. The vibration absorbing member is formed from a ceramic materialsimilarly to the tie rod 8 described above.

With a conventional complex motion drive apparatus constructed asdescribed above, the ball screw nut 53 and the ball spline nut 54 areeach rotated separately and independently by using two motors (notshown). By combining rotational motion of the ball screw nut 53 androtational motion of the ball spline nut 54, stroking motion, rotationalmotion, and spiraling motion in which stroke and rotational motions arecombined, of the output shaft 50 can be obtained. For example, when theball screw nut 53 is rotated in a state where the ball spline nut 54 isat rest, stroking motion is obtained from the output shaft 50 accordingto the rotation direction of the ball screw nut 53. Further, when theball spline nut 54 is rotated in a state where the ball screw nut 53 isat rest, spiraling motion that coincides with the rotation direction ofthe ball spline nut 54 is obtained from the output shaft 50.

When stroking motion, rotational motion, or spiraling motion is impartedto the robot arm, the plurality of balls that are provided to the ballscrew nut 53 and the ball spline nut 54 roll in the ball spline groove52 and in the ball screw groove 51 of the output shaft 50. Accordingly,the output shaft 50 is vibrated by the rotation, and further, vibrationoccurs accompanying rotation of the output shaft 50 itself. When theball screw nut 53 and the ball spline nut 54 are rotated at high speedof the like, vibration therefore remains in the output shaft 50 afterpositioning is performed on the output shaft 50 that has completed apredetermined motion. Robot operations in a stopped position thus cannotbe performed until the vibrations attenuate, and there is a problem inthat the tact time cannot be shortened.

With the robot arm of this embodiment, however, the ceramic vibrationabsorbing member fills the inside of the hollow portion of the outputshaft 50. Accordingly, vibrations that occur in the output shaft due toball rolling or due to high speed rotation of the output shaft itselfcan be attenuated at an early stage. It thus becomes possible to shortenthe tact time for assembly operations and the like that use the robotarm.

FIG. 6 is a diagram that shows a screw shaft 80 of a ball screwapparatus applying the present invention.

The tie rod 8 shown in FIG. 1 and the output shaft 50 of the robot armshown in FIG. 4 are both formed as hollow shafts having openings at bothshaft ends, and the vibration absorbing member is filled into the hollowportions from the openings at both end portions. However, it isnecessary to mount bearings that support rotation of a screw shaft atboth shaft ends of the screw shaft of the ball screw apparatus. Further,it is necessary to mount couplings for transmitting rotation of a motorto the screw shaft. There are thus many cases where the shaft ends mustbe machined into certain shapes according to intended applications.

Therefore, with the screw shaft 80 shown in FIG. 6, a vibrationabsorbing member 84 is filled within a hollow portion 83 of a hollowshaft 82 in which a helical ball rolling groove 81 is formed in an outercircumferential surface of the hollow shaft 82. Terminal portions 85 and86 that have been machined into a predetermined shape are then bonded toopenings at both ends of the hollow shaft 82, thus blocking theopenings. Friction welding is used in bonding the terminal portions 85and 86 to the shaft ends of the hollow shaft 82. Specifically, in astate where the shaft centers of the hollow shaft 82 and the terminalportions 85 and 86 are made to coincide, the hollow shaft 82 is fixed,while the terminal end portions 85 and 86 are rotated at high speed,causing the terminal portions 85 and 86 to be pressure welded to theaxial end surfaces of the hollow shaft 82.

The terminal portions 85 and 86 can thus be provided to both shaft endsof the hollow shaft 82 that is filled with the vibration absorbingmember 84 made from a ceramic material. It becomes possible to producethe screw shaft 80 having a large damping ratio with respect tovibration.

Industrial Applicability

As explained above, according to the shaft member of the presentinvention that is provided with a vibration damping function, it ispossible to positively attenuate vibrations acting on a shaft member,and therefore to suppress noise generation as much as possible, byfilling the vibration absorbing member made from a ceramic materialwithin the hollow portion of the shaft member without leaving any gaps.

1) A shaft member characterized by comprising a shaft main body made ofmetal and provided with a hollow portion, and a ceramic material servingas a vibration absorbing member and filling the hollow portion with nogaps: 2) A shaft member according to claim 1, characterized in that theceramic material is filled within the hollow portion of the shaft memberin a powder, granular, or fluid state, and formed within the hollowportion into a shape in conformity with the hollow portion. 3) A shaftmember according to claim 1, characterized in that the ceramic materialis a hydraulic composition having a hydraulic powder and a non-hydraulicpowder as its main components. 4) A shaft member according to claim 1,characterized in that opening portions at both ends of the shaft mainbody are closed by terminal portions that are bonded to the openingportions, sealing the vibration absorbing member within the hollowportion of the shaft member.